This invention relates to an element for use in a flash memory.
The most commonly used non-volatile semiconductor memory device is the flash memory device. Such a device is capable of both being written to and erased. A popular configuration of the device consists of a silicon substrate with an insulating dielectric formed thereon. An isolated conducting gate is formed on the insulating dielectric with a second conducting gate placed on an insulating layer above the whole structure. The substrate is usually p-type material. By providing the second gate with a positive potential the device becomes conductive, as electrons are attracted to the interface between the insulator and the silicon substrate. The voltage at which the device turns on is usually referred to as the threshold voltage of the device, and is dependent upon the amount of charge stored in the insulator, or floating gate.
The charge stored on the floating gate is adjusted by injecting electrons from the silicon substrate to the gate. This is achieved either by providing a large source-drain voltage which generates hot carriers which can tunnel more easily into the gate or by using Fowler Nordheim tunnelling through a very thin insulating oxide layer. The problem with the former technique is that only a small fraction of the current runs into the floating gate, therefore requiring large currents to program the gate as a whole, making the device very power greedy. Another problem with the former approach is that the programming of a large number of elements takes a long time because any metallised supply leads can only take a limited amount of current. The latter approach also has a number of problems, in particular in relation to the need to grow a very thin insulating oxide layer that does not allow tunnelling when power is off. Furthermore, it is desirable for the charge on the floating gate to be stored over very long periods, but the provision of a thin oxide insulation means that this is difficult to achieve.
Another problem associated with either approach is that tunnelling of electrons results in a degredation of the properties of the insulating oxide, and also in charge being stored in the oxide. Eventually, after perhaps as few as 100,000 programming cycles, the device fails. Even before this the threshold voltage will start to shift from expected values.
With the above-described flash memory element charge is removed from the floating gate by providing a negative bias to the second top gate, causing electrons to tunnel through the insulator to the silicon substrate, or by ensuring holes are injected from the second top gate to the floating gate to reduce the charge stored thereon. This process can be very slow.
With this type of element the charge stored on the floating gate may represent a binary value, as determined by whether or not the value of the charge exceeds a predetermined level. In recent times, however, devices which represent more than a single binary value, employing several threshold levels, have been provided. With these last devices the need to provide an accurate amount of charge and to prevent leakage has become particularly important.
According to the present invention there is provided a memory element for a flash memory comprises a first element made from electrically conductive material and having a stable position, an electrically conductive floating gate for storing charge, and means for urging the element from its stable position towards a position in which it enables the transfer of charge to the floating gate.
The charge may be transferred from the first element to the floating gate or may be transferred from a second element which contacts the floating gate when the first element is in its charge transfer position. The element may be formed on a semiconductor (such as silicon) substrate, with the floating gate mounted thereon, separated by an insulating layer. Alternatively, the floating gate may be mounted on the first element, with an insulating layer positioned therebetween.
The first element may be urged into its charge transfer position by an electric field generated by the semiconductor substrate or by a dedicated switching electrode.
A flash memory device comprising the above-defined element is also provided, as is a corresponding method of storing electrical charge.
The present invention provides an element which can be programmed and erased very quickly. In many embodiments it also avoids the need for the provision of a thin oxide insulating layer.